In the manufacture of semiconductors and other silicon wafer devices such as photovoltaic cells, wafers are frequently generated which cannot be processed into finished product. Sometimes these wafers are process rejects, other times they may be calibration wafers used to set or verify manufacturing process parameters. Reclamation of such wafers, even if only to allow reuse in process calibration, can provide significant cost savings in the manufacturing process. To accomplish reclamation, multiple layers of different materials may have to be removed. At least one of the layers is typically a silicon oxide layer. Mechanical polishing can be employed but it is expensive and time consuming. In some cases chemical stripping using multiple chemical solutions can be effectively employed to strip a wafer back to a mirror finish silicon surface. However, in very many cases the chemically stripped wafer is left with a hazy appearance which renders it unsuitable for reuse. The haze is resistant to removal by chemicals which do not attack the underlying silicon. The problem is particularly prevalent when an overlying layer contains aluminum, although aluminum has not been identified in the haze itself. In multilayer stacks, the presence of aluminum seems to be associated with haze formation even though the aluminum is many layers above the silicon wafer surface. Since aluminum metal lines are ubiquitous on semiconductor devices, this haze is a serious problem for chemical wafer reclamation. The aluminum used in semiconductors typically contains trace levels of copper and silicon.
U.S. Pat. No. 4,261,791, Shwartzman (RCA Corp.), discloses a method of cleaning silicon wafers. In the first step, following a water rinse, the wafers are subjected to a film forming solution comprising 99.1% to 99.5% by volume of a 49% HF solution by weight and 0.5% to 0.9% by volume of a 70% HNO.sub.3 solution by weight. In the second step, the wafers are subject to a stripping solution comprising 1% to 5% by volume of the HF solution and 95% to 99% by volume of the HNO.sub.3 solution. This reference does not disclose a process to reduce or prevent haze formation on a substrate.
U.S. Pat. No. 5,656,097, Olesen et al. (Verteq, Inc.), discloses a method for cleaning of semiconductor wafers in which the wafers are subjected to sequential flows of highly dilute cleaning solution. The sequence generally involves a first treatment with a solution comprising H.sub.2 O/H.sub.2 O.sub.2 and NH.sub.4 OH, followed by a second treatment using a buffered oxide etching solution such as H.sub.2 O/HF. A third treatment, again comprising H.sub.2 O/H.sub.2 O.sub.2 and NH.sub.4 OH, is followed by a fourth treatment comprising HCl. Again, the reference does not disclose a process to reduce or prevent haze formation on a substrate.
U.S. Pat. No. 3,923,567, Lawrence (Silicon Materials, Inc.), discloses a method of reclaiming a semiconductor wafer. The method involves drawing impurities to the wafer surface and chemical etching to remove point defects, grinding the back surface of the wafer and finally polishing the front of the wafer.
U.S. Pat. No. 4,891,325, Hezel et. al. (Nukem GmbH), discloses a method for reusing silicon base material of defective MIS inversion-layer solar cells using mineral acids such as O--H.sub.3 PO.sub.4 to remove an insulating layer and a electrically conductive contact. The reference does not disclose a process for the prevention or removal of haze.
U.S. Pat. No. 4,778,532, McConnell et al. (CFM Technologies Limited Partnership), discloses an apparatus and method for reducing contamination of wafers. The apparatus and method do not require handling of the wafers between processing steps. The reference contemplates the use of acids, but it does not disclose a process for removing haze in wafer reclamation.
U.S. Pat. No. 4,009,299, Flowers et al. (Motorola, Inc.), discloses a process for reclamation of semiconductor devices in which defective tin plated leads are removed. The process is not applied to the prevention or removal of haze.
U.S. Pat. No. 5,626,715, Rostoker (LSI Logic Corporation), discloses a method of polishing semiconductor substrates. The method is directed to the use of a polishing pad rather than the chemical removal of contaminants.